Landfills generate leachate due to liquids present in the wastes and rainwater percolating into the waste. Unless properly contained, leachate contaminates groundwater under the landfill, and the contaminated groundwater flows farther and contaminates larger downgradient areas. Most landfills of earlier days did not have a proper leachate containment feature and contaminated groundwater. Since the late 1970s, federal and state regulations required landfills to have a bottom liner to minimize leachate releases. The bottom liner is an impermeable or low permeability material covering the entire bottom area of the landfill. For proper leachate containment, the leachate accumulating on the bottom liner should be removed. Therefore, the bottom liner almost always includes a leachate recovery feature. With time, the regulations became tighter and required multiple layers of liners and leachate recovery features. A typical bottom liner system required for modern landfills is shown in FIG. 1 and consists of the following components below a waste layer 50:
Soil layer 51--This is a cushion layer protecting the bottom liner components below from earthwork equipment during waste placement. PA1 Geotextile layer 52--Geotextile is a textile made of synthetic polymer fibers and used for construction applications. Geotextile layer 52 used here separates the overlying soil layer and underlying sand layer while allowing liquid to flow through. PA1 First Sand/Pipe layer 53--This layer is designed to remove leachate accumulating on the first layer of Flexible Membrane Liner (FML) 54. Therefore, this layer is called LCRS (Leachate Collection and Recovery System) and consists of permeable materials and drain pipes. PA1 First FML layer 54--This is the first layer of FML. FML is a polymeric membrane sheet and almost impermeable, forming a good barrier to liquid flow. The use of an FML layer for liquid containment is very common. PA1 Second Sand/Pipe layer 55--This layer is designed to remove leachate leaking through first FML layer 54 and then accumulating on second FML layer 56. This layer is called LDRS (Leachate Detection and Recovery System) and is very similar to LCRS. PA1 Second FML layer 56--This is the second FML barrier designed to contain liquid leaking through first FML layer 54. PA1 Clay layer 57--This layer is the third barrier designed to reduce leachate leakage through second FML layer 56. An FML layer and a clay layer joined together as shown in FIG. 1 are often called a composite liner. PA1 Barrier walls--Barrier walls are any of low-permeability, vertical walls installed to cut off horizontal flow of contaminated groundwater. Types of barrier walls include soil-bentonite trench walls, vibrated-beam walls, sheetpile walls, and membrane walls. They have been used to contain contaminated groundwater from old waste sites or spill sites. Barrier walls are often used with a trench drain or a series of wells designed to recover contaminated groundwater from the area enclosed by the barrier wall. PA1 Underdrain--Underdrains are permeable materials and pipes placed beneath earthen materials (e.g., soil, rock, ore pile, tailings) to recover liquids flowing through the earthen material. An underdrain is similar to the sand/pipe layer 53 of the bottom liner system if installed on a bottom liner. If used without a bottom liner, it cannot stop downward leakage of liquids; it only recovers a portion of liquids. PA1 Trench drain--Trench drains are in common use to lower the water table or to recover groundwater. Typical construction includes a narrow trench extending below the water table, a drain pipe installed at the bottom of the trench, gravel or other permeable material filling the trench, and geotextile enveloping gravel to keep the surrounding soil from clogging gravel.
Intact Flexible Membrane Liners FMLs) have good liquid containment properties. However, FMLs in service have some defects (e.g., holes, tears, slits, open seams, etc.) developed during manufacturing or field installation. Leachate can leak through these defects. In the system shown in FIG. 1, second FML layer 56 and second sand/pipe layer 55 (i.e., LDRS) are used to contain the leachate leaking through first FML layer 54. Since clay has a low permeability, clay layer 57 below second FML layer 56 can reduce leachate leakage through the defects in second FML layer 56. The bottom liner system of FIG. 1 consists of a double liner and a double leachate recovery layer. Depending on the level of protection required, a bottom liner system may include a single liner and a single leachate recovery layer, triple liners and triple leachate recovery layers, or other variations.
As shown in FIG. 2, a conventional bottom liner system 60 is installed at the bottom of the landfill waste 50 and substantially above the groundwater table 61 to protect groundwater. In this setting, leachate accumulating on the liner has a tendency to flow downward, as shown by leachate flow direction arrows 62, regardless of the level of protection provided. Most of the leachate is withdrawn and delivered to leachate treatment facility 5.
Other prior art related to this invention includes the following: